Destruction of ozone layer is now regarded as a serious problem, such that it has been decided to terminate use of several fluorocarbon gases which destruct ozone within the next several years. Because of this, fluorocarbon gases to be used as coolants of refrigerating machines, air conditioners and the like are now being changed from CFC (fluorocarbons comprised of three elements C, F and Cl) such as R-11 (CCl.sub.3 F) and R-12 (CCl.sub.2 F.sub.2), HCFC (fluorocarbons comprised of four elements C, H, Cl and F) such as R-22 (CHClF.sub.2) and R-123 (CHClFCClF.sub.2), and HFC (fluorocarbons comprised of three elements C, H and F; ozone destruction coefficient, 0) such as R-134a (CH.sub.2 FCF.sub.3) and R-407c (CH.sub.2 F.sub.2 /CF.sub.3 CHF.sub.2 /CH.sub.2 FCF.sub.3 mixture (23/25/52 wt %)). In response to this, refrigerating machine oils to be used (lubricating oils of refrigerating machines) are also changing from mineral oils used in combination with the old fluorocarbons to oils of ester type, polyalkylene glycol type, etc.
With such substitution of coolants and refrigerating machine oils, rubber products suited for these substitutes have been called for. The conventionally used fluoroelastmer is eroded by the new fluorocarbon R-134a. Also, chloroprene rubber accelerates hydrolysis of ester oil and deteriorates refrigerating machine oil. As a consequence, vulcanized rubber compositions based on nitrile rubber or chlorinated polyethylene have been examined as the rubber for use with the new fluorocarbons.
For example, JP-A-3-250037 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes a vulcanized rubber composition which contains nitrile rubber, chlorinated polyethylene and an organic peroxide, JP-A-5-140334 proposes a rubber molding for substitute fluorocarbons use which comprises chlorinated rubber (chlorinated polyethylene, chloroprene rubber or the like) and nitrile rubber, and JP-A-6-128415 proposes a blend rubber composition in which an organic peroxide is added to a blend of nitrile rubber and chlorinated ethylene-propylene copolymer rubber.
However, when a vulcanized rubber composition containing chlorinated rubber is used in a refrigerating machine oil, chlorine as a component thereof acts as a catalyst to accelerate hydrolysis of the refrigerating machine oil, thereby causing a problem of reducing lubricity due to reduced viscosity. Such a tendency becomes particularly significant when fluorocarbon is present or the refrigerating machine oil is an ester oil. Though JP-A-5-140334 describes that a metal oxide, a metal hydroxide, a metallic soap and the like are contained in a vulcanized rubber composition, these metal compounds further accelerate hydrolysis of ester oil.
In addition, JP-B-5-17930 (the term "JP-B" as used herein means an "examined Japanese patent publication") discloses a vulcanized rubber composition for O-ring molding use in which 10 to 150 parts by weight of carbon black is added to 100 parts by weight of hydrogenated nitrile rubber, and JP-A-5-65369 discloses a vulcanized rubber composition which comprises hydrogenated nitrile rubber having a Mooney viscosity of 100 or more (ML.sub.1+4 (100.degree. C.)) and an organic peroxide. However, even the vulcanized rubber compositions produced by these techniques are not always satisfactory with regard to their resistance to fluorocarbons and refrigerating machine oils and to deterioration of refrigerating machine oils.